Technical Field
The present disclosure relates to methods for performing a random access procedure between a user equipment and a radio base station in a mobile communication system. The present disclosure is also providing the user equipment and a radio base station for participating in the method(s) described herein.
Description of the Related Art
Long Term Evolution (LTE)
Third-generation mobile systems (3G) based on WCDMA radio-access technology are being deployed on a broad scale all around the world. A first step in enhancing or evolving this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), giving a radio access technology that is highly competitive.
In order to be prepared for further increasing user demands and to be competitive against new radio access technologies, 3GPP introduced a new mobile communication system which is called Long Term Evolution (LTE). LTE is designed to meet the carrier needs for high speed data and media transport as well as high capacity voice support for the next decade.
The work item (WI) specification on Long-Term Evolution (LTE) called Evolved UMTS Terrestrial Radio Access (UTRA) and evolved UMTS Terrestrial Radio Access Network (UTRAN) is finalized as Release 8 (LTE Rel. 8). The LTE system represents efficient packet-based radio access and radio access networks that provide full IP-based functionalities with low latency and low cost. In LTE, scalable multiple transmission bandwidths are specified such as 1.4, 3.0, 5.0, 10.0, 15.0, and 20.0 MHz, in order to achieve flexible system deployment using a given spectrum. In the downlink, Orthogonal Frequency Division Multiplexing (OFDM)-based radio access was adopted because of its inherent immunity to multipath interference (MPI) due to a low symbol rate, the use of a cyclic prefix (CP) and its affinity to different transmission bandwidth arrangements. Single-carrier frequency division multiple access (SC-FDMA)-based radio access was adopted in the uplink, since provisioning of wide area coverage was prioritized over improvement in the peak data rate considering the restricted transmit power of the user equipment (UE). Many key packet radio access techniques are employed including multiple-input multiple-output (MIMO) channel transmission techniques and a highly efficient control signaling structure is achieved in LTE Rel. 8/9.
LTE Architecture
The overall LTE architecture is shown in FIG. 1. The E-UTRAN consists of an eNodeB, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the user equipment (UE). The eNodeB (eNB) hosts the Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC) and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header compression and encryption. It also offers Radio Resource Control (RRC) functionality corresponding to the control plane. It performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated uplink Quality of Service (QoS), cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of downlink/uplink user plane packet headers. The eNodeBs are interconnected with each other by means of the X2 interface.
The eNodeBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (SGW) by means of the S1-U. The S1 interface supports a many-to-many relation between MMEs/Serving Gateways and eNodeBs. The SGW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNodeB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PDN GW). For idle-state user equipments, the SGW terminates the downlink data path and triggers paging when downlink data arrives for the user equipment. It manages and stores user equipment contexts, e.g., parameters of the IP bearer service, or network internal routing information. It also performs replication of the user traffic in case of lawful interception.
The MME is the key control-node for the LTE access-network. It is responsible for idle-mode user equipment tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the SGW for a user equipment at the initial attach and at the time of intra-LTE handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the HSS). The Non-Access Stratum (NAS) signaling terminates at the MME, and it is also responsible for the generation and allocation of temporary identities to user equipments. It checks the authorization of the user equipment to camp on the service provider's Public Land Mobile Network (PLMN) and enforces user equipment roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming user equipments.
Component Carrier Structure in LTE
The downlink component carrier of a 3GPP LTE system is subdivided in the time-frequency domain in so-called subframes. In 3GPP LTE each subframe is divided into two downlink slots as shown in FIG. 2, wherein the first downlink slot comprises the control channel region (PDCCH region) within the first OFDM symbols. Each subframe consists of a give number of OFDM symbols in the time domain (12 or 14 OFDM symbols in 3GPP LTE (Release 8)), wherein each OFDM symbol spans over the entire bandwidth of the component carrier. The OFDM symbols thus each consist of a number of modulation symbols transmitted on respective subcarriers. In LTE, the transmitted signal in each slot is described by a resource grid of NRBDLNscRB subcarriers and NsymbDL OFDM symbols. NRBDL is the number of resource blocks within the bandwidth. The quantity NRBDL depends on the downlink transmission bandwidth configured in the cell and shall fulfill NRBmin,DL≤NRBDL≤NRBmax,DL, where NRBmin,DL=6 and NRBmax,DL=110 are respectively the smallest and the largest downlink bandwidths, supported by the current version of the specification. NscRB is the number of subcarriers within one resource block. For normal cyclic prefix subframe structure, NscRB=12 and NsymbDL=7.
Assuming a multi-carrier communication system, e.g., employing OFDM, as for example used in 3GPP Long Term Evolution (LTE), the smallest unit of resources that can be assigned by the scheduler is one “resource block”. A physical resource block
(PRB) is defined as consecutive OFDM symbols in the time domain (e.g., 7 OFDM symbols) and consecutive subcarriers in the frequency domain as exemplified in FIG. 2 (e.g., 12 subcarriers for a component carrier). In 3GPP LTE (Release 8), a physical resource block thus consists of resource elements, corresponding to one slot in the time domain and 180 kHz in the frequency domain (for further details on the downlink resource grid, see for example 3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)”, current version 12.6.0, section 6.2, available at http://www.3gpp.org and incorporated herein by reference).
One subframe consists of two slots, so that there are 14 OFDM symbols in a subframe when a so-called “normal” CP (cyclic prefix) is used, and 12 OFDM symbols in a subframe when a so-called “extended” CP is used. For sake of terminology, in the following the time-frequency resources equivalent to the same consecutive subcarriers spanning a full subframe is called a “resource block pair”, or equivalent “RB pair” or “PRB pair”. The term “component carrier” refers to a combination of several resource blocks in the frequency domain. In future releases of LTE, the term “component carrier” is no longer used; instead, the terminology is changed to “cell”, which refers to a combination of downlink and optionally uplink resources. The linking between the carrier frequency of the downlink resources and the carrier frequency of the uplink resources is indicated in the system information transmitted on the downlink resources.
Similar assumptions for the component carrier structure will apply to later releases too.
Carrier Aggregation in LTE-A for Support of Wider Bandwidth
The frequency spectrum for IMT-Advanced was decided at the World Radio communication Conference 2007 (WRC-07). Although the overall frequency spectrum for IMT-Advanced was decided, the actual available frequency bandwidth is different according to each region or country. Following the decision on the available frequency spectrum outline, however, standardization of a radio interface started in the 3rd Generation Partnership Project (3GPP).
The bandwidth that the LTE-Advanced system is able to support is 100 MHz, while an LTE system can only support 20 MHz. Nowadays, the lack of radio spectrum has become a bottleneck of the development of wireless networks, and as a result it is difficult to find a spectrum band which is wide enough for the LTE-Advanced system. Consequently, it is urgent to find a way to gain a wider radio spectrum band, wherein a possible answer is the carrier aggregation functionality.
In carrier aggregation, two or more component carriers are aggregated in order to support wider transmission bandwidths up to 100 MHz. Several cells in the LTE system are aggregated into one wider channel in the LTE-Advanced system which is wide enough for 100 MHz even though these cells in LTE may be in different frequency bands. All component carriers can be configured to be LTE Rel. 8/9 compatible, at least when the bandwidth of a component carrier does not exceed the supported bandwidth of an LTE Rel. 8/9 cell. Not all component carriers aggregated by a user equipment may necessarily be Rel. 8/9 compatible. Existing mechanisms (e.g., barring) may be used to avoid Rel. 8/9 user equipments to camp on a component carrier.
A user equipment may simultaneously receive or transmit on one or multiple component carriers (corresponding to multiple serving cells) depending on its capabilities. An LTE-A Rel. 10 user equipment with reception and/or transmission capabilities for carrier aggregation can simultaneously receive and/or transmit on multiple serving cells, whereas an LTE Rel. 8/9 user equipment can receive and transmit on a single serving cell only, provided that the structure of the component carrier follows the Rel. 8/9 specifications.
Carrier aggregation is supported for both contiguous and non-contiguous component carriers with each component carrier limited to a maximum of 110 Resource Blocks in the frequency domain (using the 3GPP LTE (Release 8/9) numerology).
It is possible to configure a 3GPP LTE-A (Release 10)-compatible user equipment to aggregate a different number of component carriers originating from the same eNodeB (base station) and of possibly different bandwidths in the uplink and the downlink. The number of downlink component carriers that can be configured depends on the downlink aggregation capability of the UE. Conversely, the number of uplink component carriers that can be configured depends on the uplink aggregation capability of the UE. It may currently not be possible to configure a mobile terminal with more uplink component carriers than downlink component carriers. In a typical TDD deployment the number of component carriers and the bandwidth of each component carrier in uplink and downlink is the same. Component carriers originating from the same eNodeB need not provide the same coverage.
The spacing between center frequencies of contiguously aggregated component carriers shall be a multiple of 300 kHz. This is in order to be compatible with the 100 kHz frequency raster of 3GPP LTE (Release 8/9) and at the same time to preserve orthogonality of the subcarriers with 15 kHz spacing. Depending on the aggregation scenario, then n×300 kHz spacing can be facilitated by insertion of a low number of unused subcarriers between contiguous component carriers.
The nature of the aggregation of multiple carriers is only exposed up to the MAC layer. For both uplink and downlink there is one HARQ entity required in MAC for each aggregated component carrier. There is (in the absence of SU-MIMO for uplink) at most one transport block per component carrier. A transport block and its potential HARQ retransmission(s) need to be mapped on the same component carrier.
When carrier aggregation is configured, the mobile terminal only has one RRC connection with the network. At RRC connection establishment/re-establishment, one cell provides the security input (one ECGI, one PCI and one ARFCN) and the non-access stratum mobility information (e.g., TAI) similarly as in LTE Rel. 8/9. After RRC connection establishment/re-establishment, the component carrier corresponding to that cell is referred to as the downlink Primary Cell (PCell). There is always one and only one downlink PCell (DL PCell) and one uplink PCell (UL PCell) configured per user equipment in connected state. Within the configured set of component carriers, other cells are referred to as Secondary Cells (SCells); with carriers of the SCell being the Downlink Secondary Component Carrier (DL SCC) and Uplink Secondary Component Carrier (UL SCC). Maximum five serving cells, including the PCell, can be configured at the moment for one UE.
The configuration and reconfiguration, as well as addition and removal, of component carriers can be performed by RRC. Activation and deactivation is done, e.g., via MAC control elements. At intra-LTE handover, RRC can also add, remove, or reconfigure SCells for usage in the target cell. When adding a new SCell, dedicated RRC signaling is used for sending the system information of the SCell, the information being necessary for transmission/reception (similarly as in Rel. 8/9 for handover). Each SCell is configured with a serving cell index, when the SCell is added to one UE; PCell has always the serving cell index 0.
When a user equipment is configured with carrier aggregation there is at least one pair of uplink and downlink component carriers that is always active. The downlink component carrier of that pair might be also referred to as ‘DL anchor carrier’. Same applies also for the uplink. When carrier aggregation is configured, a user equipment may be scheduled on multiple component carriers simultaneously, but at most one random access procedure shall be ongoing at any time. Cross-carrier scheduling allows the PDCCH of a component carrier to schedule resources on another component carrier. For this purpose a component carrier identification field is introduced in the respective DCI (Downlink Control Information) formats, called CIF.
A linking, established by RRC signaling, between uplink and downlink component carriers allows identifying the uplink component carrier for which the grant applies when there is no cross-carrier scheduling. The linkage of downlink component carriers to uplink component carrier does not necessarily need to be one to one. In other words, more than one downlink component carrier can link to the same uplink component carrier. At the same time, a downlink component carrier can only link to one uplink component carrier.
Random Access Procedure
A mobile terminal in LTE can only be scheduled for uplink transmission, if its uplink transmission is time synchronized so as to maintain orthogonality with uplink transmissions from other UEs. The Random Access (RACH) procedure plays an important role as an interface between non-synchronized mobile terminals (UEs) and the orthogonal transmission of the uplink radio access. Essentially, the Random Access procedure in LTE is used to achieve uplink time synchronization for a user equipment which either has not yet acquired or has lost its uplink synchronization. Once a user equipment has achieved uplink synchronization, the eNodeB can schedule uplink transmission resources for it.
PRACH transmission and detection also provides an estimation of the round-trip delay between the eNB and the UE. The design target regarding the PRACH signal shape for licensed band LTE operation was minimization of overhead and interference impact on parallel uplink transmissions from other UEs while providing at the same time sufficient round-trip delay estimation accuracy.
There is one more additional case where a user equipment performs a random access procedure, even though the user equipment is time-synchronized, namely when the user equipment uses the random access procedure in order to send a scheduling request, i.e., uplink buffer status report, to its eNodeB, in case it does not have any other uplink resource(s) allocated in which to send the scheduling request, e.g., dedicated scheduling request (D-SR) channel is not configured.
The following scenarios are therefore relevant for random access:                1. A user equipment in RRC_CONNECTED state, but not uplink-synchronized, wishing to send new uplink data or control information        2. A user equipment in RRC_CONNECTED state, but not uplink-synchronized, required to receive downlink data, and therefore to transmit corresponding HARQ feedback, i.e., ACK/NACK, in the uplink. This scenario is also referred to as Downlink data arrival        3. A user equipment in RRC_CONNECTED state, handing over from its current serving cell to a new target cell; in order to achieve uplink time-synchronization in the target cell, Random Access procedure is performed        4. For positioning purposes in RRC_CONNECTED state, when timing advance is needed        5. A transition from RRC_IDLE state to RRC CONNECTED, for example for initial access or tracking area updates        6. Recovering from radio link failure, i.e., RRC connection re-establishment        
LTE offers two types of random access procedures allowing the access to be either contention based (implying an inherent risk of collision) or contention-free. It should be noted that contention-based random access can be applied for all six scenarios listed above, whereas a contention-free random access procedure can only be applied for the downlink data arrival and handover scenario.
In the following the contention-based random access procedure is being described in more detail with respect to FIG. 3. A detailed description of the random access procedure can be also found in 3GPP TS 36.321, current version 12.6.0, section 5.1, incorporated herein by reference.
FIG. 3 shows the contention-based RACH procedure of LTE. This procedure consists of four “steps”. First, the user equipment transmits 301 a random access preamble on the Physical Random Access Channel (PRACH) to the eNodeB. The preamble is selected by the user equipment from a set of available random access preambles reserved by eNodeB for contention-based access; Nd is the number of signatures reserved by the eNodeB for contention-free RACH. In LTE, there are 64 preambles in total per cell which can be used for contention-free as well as contention-based random access. The set of contention-based preambles can be further subdivided into two groups, so that the UE's choice of preamble can carry one bit of information to indicate information relating to the amount of transmission resources needed for the first scheduled transmission, which is referred to as msg3 in TS 36.321 (see step 303 in FIG. 3). The system information broadcasted in the cell contains the information which signatures (preambles) are in each of the two subgroups as well as the meaning of each subgroup. The user equipment randomly selects one preamble from the subgroup corresponding to the size of transmission resource needed for the msg3-transmission (see later step 303). When selecting the appropriate size to indicate, the UE may additionally take into account the current downlink path-loss and the required transmission power for the step 303 message in order to avoid being granted resources for a message size that would need a transmission exceeding that which the UE's maximum power would allow.
After the eNodeB has detected a RACH preamble, it sends 302 a Random Access Response (RAR) message on the PDSCH (Physical Downlink Shared Channel), the corresponding DCI on the PDCCH being addressed to the (Random Access) RA-RNTI that identifies the time-frequency slot in which the preamble was detected. If multiple user equipments transmitted the same RACH preamble in the same PRACH resource, which is also referred to as collision, they would receive the same random access response.
The RAR message conveys the identity of the detected RACH preamble, a timing alignment command (TA command) for synchronization of subsequent uplink transmissions, an initial uplink resource assignment (grant) for the transmission of the first scheduled transmission (see step 303) and an assignment of a Temporary Cell Radio Network Temporary Identifier (T-CRNTI). This T-CRNTI is used by eNodeB in order to address the mobile(s) whose RACH preamble were detected until RACH procedure is finished, since the “real” identity of the mobile is at this point not yet known to the eNodeB.
Furthermore, the RAR message can also contain a so-called back-off indicator, which the eNodeB can set to instruct the user equipment to back off for a period of time before retrying a random access attempt. The user equipment monitors the PDCCH for reception of the random access response within a given time window, which is configured by the eNodeB. In case the user equipment does not receive a random access response within the configured time window, it retransmits the preamble at the next PRACH opportunity considering a potential back off period.
In response to the RAR message received from the eNodeB, the user equipment transmits 303 the first scheduled uplink transmission on the uplink resources assigned by the grant within the random access response. This scheduled uplink transmission conveys the actual random access procedure message like for example an RRC connection request, a tracking area update or a buffer status report. Furthermore, it includes either the C-RNTI for user equipments in RRC CONNECTED mode or the unique 48-bit user equipment identity if the user equipments are in RRC IDLE mode. In case of a preamble collision having occurred in step 301, i.e., multiple user equipments have sent the same preamble on the same PRACH resource, the colliding user equipments will receive the same T-CRNTI within the random access response and will also collide in the same uplink resources when transmitting 303 their scheduled transmission. This may result in interference such that no transmission from a colliding user equipment can be decoded at the eNodeB, and the user equipments will restart the random access procedure after having reached the maximum number of retransmission for their scheduled transmission. In case the scheduled transmission from one user equipment is successfully decoded by eNodeB, the contention remains unresolved for the other user equipments. For resolution of this type of contention, the eNodeB sends 304 a contention resolution message addressed to the C-RNTI or Temporary C-RNTI, and, in the latter case, echoes the 48-bit user equipment identity contained in the scheduled transmission of step 303. In case of collision followed by a successful decoding of the message sent in step 303, the HARQ feedback (ACK) is only transmitted by the user equipment which detects its own identity, either C-RNTI or unique user equipment ID. Other UEs understand that there was a collision at step 301 and can quickly exit the current RACH procedure and start another one.
FIG. 4 is illustrating the contention-free random access procedure introduced as of 3GPP LTE Rel. 8/9. In comparison with the contention-based random access procedure, the contention-free random access procedure is simplified. The eNodeB assigns 401 the user equipment a particular preamble to use for random access so that there is no risk of collisions (i.e., multiple user equipments do not transmit the same RACH preamble). Accordingly, the user equipment is sending 402 the preamble which was signaled by eNodeB in the uplink on a suitable PRACH resource. Since the case that multiple UEs are sending the same preamble is avoided for a contention-free random access, no contention resolution is necessary, for which reason step 304 of the contention-based procedure shown in FIG. 3 can be omitted. Essentially, a contention-free random access procedure is finished after having successfully received the random access response. In case of a missing random access response, the subsequent PRACH retransmissions are initiated autonomously by the UE itself.
When carrier aggregation is configured, the first three steps of the contention-based random access procedure occur on the PCell, while contention resolution (step 304) can be cross-scheduled by the PCell.
The initial preamble transmission power setting is based on an open-loop estimation with full compensation of the path loss. This is designed to ensure that the received power of the preambles is independent of the path-loss.
The eNB may also configure an additional power offset, depending for example on the desired received SINR, the measured uplink interference and noise level in the time-frequency slots allocated to RACH preambles, and possibly on the preamble format. Furthermore, the eNB may configure preamble power ramping so that the transmission for each retransmitted preamble, e.g., in case the PRACH transmission attempt was not successfully, is increased by a fixed step.
Random Access Preamble—Time, Frequency, Formats
The random access preamble transmission part of the random access procedure described above is mapped at the physical layer onto the PRACH. The design of the preamble is crucial to the success of the random access procedure and will be discussed in detail in the following. The RACH preamble is basically a cyclic shift of a complex Zadoff-Chu (ZC) sequence which is also known as preamble signature. The LTE PRACH preamble consists of a complex sequence. However, differing from the W-CDMA preamble, it is also an OFDM symbol having to follow the DFT-S-OFDM structure of the LTE uplink, build with a CP (cyclic prefix), thus allowing for an efficient frequency-domain receiver at the eNodeB. The physical layer random access preamble consists of a cyclic prefix of length TCP and a sequence part of length TSEQ, as illustrated in FIG. 5. Possible values for these parameters are listed in the following table and depend on the frame structure and on the random access configuration (e.g., the preamble format which can be controlled by higher layers). Corresponding detailed information can be found in the 3GPP technical standard 36.211, current version 12.6.0, chapter 5.7.1 “Time and frequency structure” incorporated herein by reference. Four random access preamble formats are defined for the frequency division duplex operation wherein each format is defined by the duration of the sequence and its cyclic prefix. The format configured in a cell is broadcast in the system information.
Pre-ambleTCPTSEQformat(μs)(μs)Typical Usage03168 · Ts24576 · TsNormal 1 ms random accessburst with 800 μs preamblesequence for small to mediumcells121024 · Ts 24576 · Ts2 ms random access burst with800 μs preamble sequence, forlarge cells without a link budgetproblem26240 · Ts2 · 24576 · Ts2 ms random access burst with1600 μs preamble sequence,for medium cells supporting lowdata rates321024 · Ts 2 · 24576 · Ts3 ms random access burst with1600 μs preamble sequence, forvery large cells4 448 · Ts 4096 · Ts2 OFDM symbol random access(seeburst with 147.6 μs preambleNote)sequence, for TDD special subframes in small cellsNOTE:Frame structure type 2 and special subframe configurations with UpPTS lengths 4384 · Ts and 5120 · Ts only.
TS is the assumed system sampling rate, which can be 1/30,72 μs and is the basic time unit in LTE. Taking this specific sampling rate into account, the following table gives the values for TCP and TSEQ for the different preamble formats.
PreambleTCPTSEQformat(us)(us)0103.338001684.388002203.1316003684.381600414.58133.33
In the following table the subcarrier spacing and the corresponding symbol duration of the current LTE specification is shown. The preamble sequence duration for, e.g., preamble formats 2 and 3 (1600 μs, see above table) are achieved by repetition of the preamble symbol (800 μs) in the time domain.
Subcarrier spacingSymbol durationTransmission type(kHz)(us)PUSCH1566.66Preamble format 0-31.25800Preamble format 47.5133.33
The lower bound (683.33 μs) for the sequence duration TSEQ must allow for unambiguous round-trip time estimation for a UE located at the edge of the largest expected cell, including the maximum delay spread expected in such large cells (namely 16.67 μs). Further constraints on the sequence duration TSEQ are given by the Single-Carrier Frequency Division Multiple Access signal generation principle, such that the size of the DFT and IDFT, NDFT, must be an integer number.
In order to ease the frequency multiplexing of the PRACH and the PUSCH resource allocations, a PRACH slot must be allocated a bandwidth BWPRACH equal to an integer multiple of resource blocks, i.e., an integer multiple of 180 kHz. For simplicity, BWPRACH in LTE (6 PRBs, 1.08 MHz) is constant for all system bandwidths; it is chosen to optimize both the detection performance and the timing estimation accuracy. The latter drives the lower bound of the PRACH bandwidth. Indeed, a minimum bandwidth of about 1 MHz is necessary to provide a one-shot accuracy of about ±0.5 μs, which is an acceptable timing accuracy for PUCCH/PUSCH transmissions.
A PRACH allocation of 6 RBs provides a good trade-off between PRACH overhead, detection performance and timing estimation accuracy. It should be noted that for the smallest system bandwidth (1.4 MHz, 6 RBs) the PRACH overlaps with the PUCCH; it is left to the eNodeB implementation whether to implement scheduling restrictions during PRACH slots to avoid collisions, or to let PRACH collide with the PUCCH and handle the resulting interference.
The preamble duration should be fixed to an integer duration of the PUSCH symbol in order to provide compatibility between preamble and PUSCH subcarriers. This means that the PRACH subcarrier spacing should preferably be a divisor of the PUSCH subcarrier spacing.
A PRACH is time- and frequency-multiplexed with the PUSCH and the PUCCH as illustrated in FIG. 6. PRACH time-frequency resources are semi-statically allocated within the PUSCH region, and repeat periodically. The possibility of scheduling PUSCH transmissions within PRACH slots is left to the eNodeB's discretion. LTE supports 64 PRACH configurations, each configuration consisting of a periodic PRACH resource pattern and an associated preamble format. A detailed listing of the PRACH configurations is given in Tables 5.7.1-2 and 5.7.1-3 of the technical standard 36.211, incorporated herein by reference. It is possible to schedule PUSCH transmissions together with allocated PRACH resources within the same subframe; the decision is made by the eNB.
Random Access Preamble—Preamble Sequence Generation
As noted above, 64 PRACH signatures are available in LTE, compared to only 16 in WCDMA. This can not only reduce the collision probability, but also allows for 1 bit of information to be carried by the preamble in the contention-based and some signatures to be reserved for contention-free access. Therefore, the LTE PRACH preamble called for an improved sequence design with respect to WCDMA. In LTE prime-length Zadoff-Chu sequences have been chosen which enable improved PRACH preamble detection performance. More detailed information can be found in the 3GPP technical standard 36.211, current version 12.6.0, chapter 5.7.2 “physical random access channel” incorporated herein by reference.
The random access preambles are Zadoff-Chu (ZC) sequences that are in turn generated from one or several root Zadoff-Chu sequences as follows. First, a root Zadoff-Chu sequence is chosen based on an indication of a logical sequence index broadcast as part of the System Information (RACH_ROOT_SEQUENCE). The logical root sequence order is cyclic such that the logical index 0 is consecutive to 837. The relation between a logical root sequence index (indicated in the system information) and a physical root sequence index u is given by Tables 5.7.2-4 and 5.7.2-5 of the technical standard 36.211 for preamble formats 0-3 and 4, respectively, incorporated herein by reference.
The u-th root Zadoff-Chu sequence is defined by:
                    x        u            ⁡              (        n        )              =          e                        -          j                ⁢                              π            ⁢                                                  ⁢                          un              ⁡                              (                                  n                  +                  1                                )                                                          N            ZC                                ,      0    ≤    n    ≤                  N        ZC            -      1      where u is the above-mentioned physical root sequence index, and wherein the sequence length NZC depends on the configured PRACH preamble format, i.e., NZC is 839 for preamble formats 0-3 and is 139 for preamble format 4 (see also Table 5.7.2-1 in TS 36.211).
From the u-th root Zadoff-Chu sequence, a set of 64 random access preambles with zero-correlation zones of length NCS-1 are defined by cyclic shifts according toxu,v(n)=xu((n+Cv)mod NZC)
The cyclic shift is given by
      C    v    =      {                                        vN            CS                                                              v              =              0                        ,            1            ,            …            ⁢                                                  ,                                          ⌊                                                      N                    ZC                                    /                                      N                    CS                                                  ⌋                            -              1                        ,                                          N                CS                            ≠              0                                                            for            ⁢                                                  ⁢            unrestricted            ⁢                                                  ⁢            sets                                                0                                                    N              CS                        =            0                                                for            ⁢                                                  ⁢            unrestricted            ⁢                                                  ⁢            sets                                                                                          d                start                            ⁢                              ⌊                                  v                  /                                      n                    shift                    RA                                                  ⌋                                      +                                          (                                  v                  ⁢                                                                          ⁢                  mod                  ⁢                                                                          ⁢                                      n                    shift                    RA                                                  )                            ⁢                              N                CS                                                                                        v              =              0                        ,            1            ,            …            ⁢                                                  ,                                                            n                  shift                  RA                                ⁢                                  n                  group                  RA                                            +                                                n                  _                                shift                RA                            -              1                                                            for            ⁢                                                  ⁢            restricted            ⁢                                                  ⁢            sets                              
The parameter NCS is given by Tables 5.7.2-2 and 5.7.2-3 in the Technical Standard 36.211, and depends on the preamble format and on the zeroCorrelationZoneConfig parameter provided by higher layers. Further information can be obtained from the technical standard 36.211, section 5.7.2.
Additional preamble sequences, in case 64 preambles cannot be generated from a single root Zadoff-Chu sequence, are obtained from one or more root sequences with consecutive logical indexes until all the 64 preamble sequences are found.
In summary, the set of 64 preamble sequences that are available for use in a cell for the RACH procedure is generated by cyclic shifts of one or more root Zadoff-Chu sequences.
Random Access Preamble—Baseband Signal Generation
The generation of the PRACH baseband signal is defined in section 5.7.3 of TS 36.211. The time-continuous random access signal s(t) is defined by
      s    ⁡          (      t      )        =            β      PRACH        ⁢                  ∑                  k          =          0                                      N            ZC                    -          1                    ⁢                        ∑                      n            =            0                                              N              ZC                        -            1                          ⁢                                            x                              u                ,                v                                      ⁡                          (              n              )                                ·                      e                                          -                j                            ⁢                                                2                  ⁢                  π                  ⁢                                                                          ⁢                  nk                                                  N                  ZC                                                              ·                      e                          j              ⁢                                                          ⁢              2              ⁢                              π                ⁡                                  (                                      k                    +                    φ                    +                                          K                      ⁡                                              (                                                                              k                            0                                                    +                                                      1                            2                                                                          )                                                                              )                                            ⁢              Δ              ⁢                                                          ⁢                                                f                  RA                                ⁡                                  (                                      t                    -                                          T                      CP                                                        )                                                                        where 0≤t<TSEQ+TCP, βPRACH is an amplitude scaling factor in order to conform to the transmit power PPRACH, k0=nPRBRANscRB−NRBULNscRB/2.
The location in the frequency domain is controlled by the parameter nPRBRA. The factor K=Δf/ΔfRA accounts for the difference in subcarrier spacing between the random access preamble and uplink data transmission. The variable ΔfRA, the subcarrier spacing for the random access preamble, and the variable φ, a fixed offset determining the frequency-domain location of the random access preamble within the physical resource blocks, are both given by the following table (see Table 5.7.3-1 in TS 36.211).
Preamble formatΔfRAφ0-31250 Hz147500 Hz2
It should be noted that PUSCH has a subcarrier spacing of 15 kHz.
The time-domain preamble sequence is transformed into the frequency domain by a DFT of size NZC. The resulting frequency-domain coefficients are mapped onto subcarriers with a frequency spacing ΔfRA. The frequency spacing for PRACH transmissions does not coincide with the frequency spacing used for other uplink transmissions, such as PUSCH or PUCCH. The subcarrier mapping further incorporates the PRACH location in the frequency domain.
FIG. 7 shows the PRACH preamble mapping onto allocated subcarriers, vis-à-vis the subcarrier mapping of PUSCH. As apparent therefrom, the PRACH uses a guard band to avoid the data interference at preamble edges. The PRACH is transmitted on a frequency-domain resource corresponding to six consecutive PRBs, i.e., with a frequency bandwidth of 1.08 MHz. These PRBs could be located at the center of the nominal system bandwidth as illustrated in FIG. 8, or could be located at any other position within the nominal system bandwidth as shown in FIG. 9.
Random Access Preamble—Preamble Sequence UE Transmitter Implementation
In the following an exemplary practical implementation of the PRACH function will be briefly explained The PRACH preamble can be generated at the system sampling rate by means of a large IDFT as illustrated in FIG. 10. The DFT block in the FIG. 10 is dashed indicating that it is optional since the sequence could also be mapped directly in the frequency domain at the IDFT input. The cyclic shift can be implemented either in the time domain after the IDFT, or in the frequency domain before the IDFT through a phase shift.
Another option for generating the preamble consists of using a smaller IDFT, actually an IFFT, and shifting the preamble to the required frequency location through time-domain upsampling and filtering. The cyclic prefix can be inserted before the upsampling and time-domain frequency shift, so as to minimize the intermediate storage requirements.
LTE on Unlicensed Bands—Licensed-Assisted Access LAA
In September 2014, 3GPP initiated a new study item on LTE operation on unlicensed spectrum. The reason for extending LTE to unlicensed bands is the ever-growing demand for wireless broadband data in conjunction with the limited amount of licensed bands. The unlicensed spectrum therefore is more and more considered by cellular operators as a complementary tool to augment their service offering. The advantage of LTE in unlicensed bands compared to relying on other radio access technologies (RAT) such as Wi-Fi is that complementing the LTE platform with unlicensed spectrum access enables operators and vendors to leverage the existing or planned investments in LTE/EPC hardware in the radio and core network.
However, it has to be taken into account that unlicensed spectrum access can never match the qualities of licensed spectrum access due to the inevitable coexistence with other radio access technologies (RATs) in the unlicensed spectrum such as Wi-Fi. LTE operation on unlicensed bands will therefore at least in the beginning be considered a complement to LTE on licensed spectrum rather than as stand-alone operation on unlicensed spectrum. Based on this assumption, 3GPP established the term Licensed Assisted Access (LAA) for the LTE operation on unlicensed bands in conjunction with at least one licensed band. Future stand-alone operation of LTE on unlicensed spectrum, i.e., without being assisted by licensed cells, however, shall not be excluded.
The currently-intended general LAA approach at 3GPP is to make use of the already specified Rel. 12 carrier aggregation (CA) framework as much as possible, where the CA framework configuration as explained before comprises a so-called primary cell (PCell) carrier and one or more secondary cell (SCell) carriers. CA supports in general both self-scheduling of cells (scheduling information and user data are transmitted on the same component carrier) and cross-carrier scheduling between cells (scheduling information in terms of PDCCH/EPDCCH and user data in terms of PDSCH/PUSCH are transmitted on different component carriers).
A very basic scenario is illustrated in FIG. 11, with a licensed PCell, licensed SCell 1, and various unlicensed SCells 2, 3, and 4 (exemplarily depicted as small cells). The transmission/reception network nodes of unlicensed SCells 2, 3, and 4 could be remote radio heads managed by the eNB or could be nodes that are attached to the network but not managed by the eNB. For simplicity, the connection of these nodes to the eNB or to the network is not explicitly shown in the figure.
At present, the basic approach envisioned at 3GPP is that the PCell will be operated on a licensed band while one or more SCells will be operated on unlicensed bands. The benefit of this strategy is that the PCell can be used for reliable transmission of control messages and user data with high quality of service (QoS) demands, such as for example voice and video, while an SCell on unlicensed spectrum might yield, depending on the scenario, to some extent significant QoS reduction due to inevitable coexistence with other RATs.
It has been agreed that the LAA will focus on unlicensed bands at 5 GHz. One of the most critical issues is therefore the coexistence with Wi-Fi (IEEE 802.11) systems operating at these unlicensed bands. In order to support fair coexistence between LTE and other technologies such as Wi-Fi as well as to guarantee fairness between different LTE operators in the same unlicensed band, the channel access of LTE for unlicensed bands has to abide by certain sets of regulatory rules which partly may depend on the geographical region and particular frequency band; a comprehensive description of the regulatory requirements for all regions for operation on unlicensed bands at 5 GHz is given in R1-144348, “Regulatory Requirements for Unlicensed Spectrum”, Alcatel-Lucent et al., RAN1#78bis, Sep. 2014 incorporated herein by reference as well as the 3GPP Technical Report 36.889, current version 13.0.0. Depending on region and band, regulatory requirements that have to be taken into account when designing LAA procedures comprise Dynamic Frequency Selection (DFS), Transmit Power Control (TPC), Listen Before Talk (LBT) and discontinuous transmission with limited maximum transmission duration. The intention of 3GPP is to target a single global framework for LAA which basically means that all requirements for different regions and bands at 5 GHz have to be taken into account for the system design.
For example, in Europe certain limits for the Nominal Channel Bandwidth is set, as apparent from section 4.3 of the European standard ETSI EN 301 893, current version 1.8.1, incorporated herein by reference. The Nominal Channel Bandwidth is the widest band of frequencies, inclusive of guard bands, assigned to a single channel. The Occupied Channel Bandwidth is the bandwidth containing 99% of the power of the signal. A device is permitted to operate in one or more adjacent or non-adjacent channels simultaneously.
When equipment has simultaneous transmissions in adjacent channels, these transmissions may be considered as one signal with an actual Nominal Channel Bandwidth of “n” times the individual Nominal Channel Bandwidth where “n” is the number of adjacent channels. When equipment has simultaneous transmissions in non-adjacent channels, each power envelope shall be considered separately. The Nominal Channel Bandwidth shall be at least 5 MHz at all times. The Occupied Channel Bandwidth shall be between 80% and 100% of the declared Nominal Channel Bandwidth. In the USA, the minimum occupied channel bandwidth is 500 kHz according to 3GPP TR 36.889. In case of smart antenna systems (devices with multiple transmit chains) each of the transmit chains shall meet this requirement. During an established communication, the device is allowed to operate temporarily with an Occupied Channel Bandwidth below 80% of its Nominal Channel Bandwidth with a minimum of 4 MHz.
The listen-before-talk (LBT) procedure is defined as a mechanism by which an equipment applies a clear channel assessment (CCA) check before using the channel. The CCA utilizes at least energy detection to determine the presence or absence of other signals on a channel in order to determine if a channel is occupied or clear, respectively.
European and Japanese regulations mandate the usage of LBT in the unlicensed bands. Apart from regulatory requirements, carrier sensing via LBT is one way for fair sharing of the unlicensed spectrum and hence it is considered to be a vital feature for fair and friendly operation in the unlicensed spectrum in a single global solution framework.
In unlicensed spectrum, channel availability cannot always be guaranteed. In addition, certain regions such as Europe and Japan prohibit continuous transmissions and impose limits on the maximum duration of a transmission burst in the unlicensed spectrum. Hence, discontinuous transmission with limited maximum transmission duration is a required functionality for LAA. DFS is required for certain regions and bands in order to detect interference from radar systems and to avoid co-channel operation with these systems. The intention is furthermore to achieve a near-uniform loading of the spectrum. The DFS operation and corresponding requirements are associated with a master-slave principle. The master shall detect radar interference, can however rely on another device, associated with the master, to implement radar detection.
The operation on unlicensed bands at 5-GHz is in most regions limited to rather low transmit power levels compared to the operation on licensed bands which results in small coverage areas. Even if the licensed and unlicensed carriers were to be transmitted with identical power, usually the unlicensed carrier in the 5 GHz band would be expected to support a smaller coverage area than a licensed cell in the 2 GHz band due to increased path loss and shadowing effects for the signal. A further requirement for certain regions and bands is the use of TPC in order to reduce the average level of interference caused for other devices operating on the same unlicensed band.
Detailed information can be found in the harmonized European standard ETSI EN 301 893, current version 1.8.0, incorporated herein by reference.
Following this European regulation regarding LBT, devices have to perform a Clear Channel Assessment (CCA) before occupying the radio channel with a data transmission. It is only allowed to initiate a transmission on the unlicensed channel after detecting the channel as free based, e.g., on energy detection. In particular, the equipment has to observe the channel for a certain minimum time (e.g., for Europe 20 μs, see ETSI 301 893, under clause 4.8.3) during the CCA. The channel is considered occupied if the detected energy level exceeds a configured CCA threshold (e.g., for Europe, −73 dBm/MHz, see ETSI 301 893, under clause 4.8.3), and conversely is considered to be free if the detected power level is below the configured CCA threshold. If the channel is determined as being occupied, it shall not transmit on that channel during the next Fixed Frame Period. If the channel is classified as free, the equipment is allowed to transmit immediately. The maximum transmit duration is restricted in order to facilitate fair resource sharing with other devices operating on the same band.
The energy detection for the CCA is performed over the whole channel bandwidth (e.g., 20 MHz in unlicensed bands at 5 GHz), which means that the reception power levels of all subcarriers of an LTE OFDM symbol within that channel contribute to the evaluated energy level at the device that performed the CCA.
Furthermore, the total time during which an equipment has transmissions on a given carrier without re-evaluating the availability of that carrier (i.e., LBT/CCA) is defined as the Channel Occupancy Time (see ETSI 301 893, under clause 4.8.3.1). The Channel Occupancy Time shall be in the range of 1 ms to 10 ms, where the maximum Channel Occupancy Time could be, e.g., 4 ms as currently defined for Europe. Furthermore, there is a minimum Idle time the UE is not allowed to transmit after a transmission on the unlicensed cell, the minimum Idle time being at least 5% of the Channel Occupancy Time. Towards the end of the Idle Period, the UE can perform a new CCA, and so on. This transmission behavior is schematically illustrated in FIG. 12, the figure being taken from ETSI EN 301 893 (there FIG. 2: “Example of timing for Frame Based Equipment”).
FIG. 13 illustrates the timing between a Wi-Fi transmission and LAA UE transmissions on a particular frequency band (unlicensed cell). As can be seen from FIG. 13, after the Wi-Fi burst, a CCA gap is at least necessary before the eNB “reserves” the unlicensed cell by, e.g., transmitting a reservation signal until the next subframe boundary. Then, the actual LAA DL burst is started.
The RACH procedure shall also be supported for unlicensed bands. It was agreed so far that only contention-free PRACH transmissions would be supported for unlicensed bands. It is still under discussion whether PRACH retransmissions will be scheduled explicitly by the eNB as well in unlicensed bands, in contrast to the PRACH retransmissions in licensed bands, as explained above. Nevertheless, even though the standardization has so far agreed that only contention-free random access shall be supported, this may change in the future and thus contention-based random access for unlicensed cells may still become relevant (actually, the principles of the disclosure are applicable to both contention-free and contention-based random access procedures).
Considering the different regulatory requirements, it is apparent that the LTE specification, among other things the random access procedure, for operation in unlicensed bands will require several changes compared to the current Rel. 12 specification that is limited to licensed band operation.